It comes in liquid form, you weigh out the parts in the ratios instructed pour both into a disposable paper container mix for 10-20 seconds. Then pour it into a mold and slush it around quickly let it foam up or cap it off.  Demold your foam piece in 1/2 an hour.  Poly foam is messy, sticky stuff and toxic when foaming, so do it with good ventilation, wear a smock and disposable gloves.  Often times you want to "skin" a prop first.  This is like making a mask.  You pour or brush latex, urethane, or silicone into the mold.  This makes the skin, it does not have to be as thick as a mask just enough for an outer skin.  Let that cure and then fill with poly foam.  In half an hour demold and you will have a rubber foam filled prop just like the ones you can buy or maybe even better.  They often demold the skin and fill it outside of the mold.  The poly foam expands, this is why you see body parts that look like balloons and other props that are "distorted".  They do this for fast mass production. If you fill inside the mold it will maintain the intended shape.  You can cast poly foam directly into silicone molds to
create pieces that are just foam if you are not worried about having a rubber surface. Rigid foams are great for making skulls in silicone molds, just pour it in, pop it out, paint and repeat. It is better than that foam insulation in a can that has to be done in layers and air dry forever! You can fabricate with poly foam too.  Mix it and pour a line on some plastic, instant guts!   I have made corpses by tinting poly foam with urethane pigments
and pouring it onto cheap skeletons and mushing it around with a gloved hand to make gore and stringy sinews. It also glues the bones in place.  I bought a bag of bones and took some sheet foam and glued it behind a ribcage.  Then I applied some poly foam gore on the top, way too gross! The possibilities are endless.